1. Field of the Invention
The present invention relates to stratified composites containing polymers which do not readily adhere to each other under the influence of heat and pressure. In particular, the present invention relates to laminar composites consisting of discrete layers of fluoropolymeric and non-fluoropolymeric materials which possess improved peel adhesive properties through the novel use of a fibrous binder.
Many commercially available polymers exhibit a wide range of useful mechanical, physical, and electrical bulk properties, and are available at an economically attractive cost. But these relatively low-cost polymers are often excluded from applications which require surface properties more typically associated with the more costly fluoropolymers, such as exceptional weatherability, flamespread inhibition, enhanced solvent resistance, durable hydrophobicity and release properties.
To obtain these enhanced fluoropolymeric surface characteristics and still benefit from the good bulk properties and economics of the lower cost polymers, it would be advantageous to create a composite by laminating a fluoropolymer to the surface of such low-cost, non-fluoropolymers. Unfortunately, however, direct thermal lamination of fluoropolymers to non-fluoropolymers in a temperature range at or above the softening or melting point of the non-fluoropolymeric material too often results in a composite with such low interfacial peel-adhesion that delamination is a commonly encountered phenomenon, severely limiting use. Such non-fluoropolymers are said to be thermally non-adherent to fluoropolymers; heat and pressure used in typical lamination processes are insufficient to develop significant interfacial peel adhesion between the components. Moreover, interfacial peel-adhesion is often so low that such a composite cannot practically be prepared.
It would, therefore, be desirable to devise a simple method for preparing a composite containing both a fluoropolymeric component and a non-fluoropolymeric component, wherein the interfacial peel-adhesion between such materials is significantly enhanced. Composites with such improved behavior would find potential use in applications, such as waste pond liners, molded gas tanks, and exterior as well as interior architectural components, such as roofs and walls.
2. Description of the Prior Art
Several approaches to laminating fluoropolymeric and non-fluoropolymeric materials which otherwise exhibit little or no peel-adhesion to each other have been attempted.
These include, for example, the use of an interposed adhesive layer as in U.S. Pat. No. 5,139,878 (Kim et al.), or the application of hot melt adhesive between them as in U.S. Pat. No. 4,677,017 and U.S. Pat. No. 4,273,815. Additives which promote adhesion, such as ferrites, are also described in Japanese Patent No. 1,193,350.
Another approach, disclosed in U.S. Pat. No. 5,108,836 (Ocampo et al.), employs a film in which a concentration gradient of a fluoropolymer blend is established across its thickness. Such a film may then be adhered via its non-fluoropolymeric-rich surface to coated fabrics by heat and pressure.
Other methods involve the use of co-polymerized or grafted functional groups to promote adhesion, such as in U.S. Pat. No. 4,749,607 (Masahide).
Yet another chemical approach to promoting adhesion includes surface modification via oxidation, reduction, or substitution (U.S. Pat. No. 4,946,903 and U.S. Pat. No. 4,740,562).
The use of synthetic woven and non-woven fabrics of PTFE and it copolymers has been described for the preparation of mechanically toughened, multilayer membranes based on perfluoro ion exchange resins (International Patent Application Publication No. 90/06337). Mechanical needling of PTFE and non-woven fabrics with fluoropolymers has also been described for laminating fluoropolymers in the manufacture of protective clothing (Japanese Patent No. 60094607 and U.S. Pat. No. 4,324,474). Composite fabrics have been prepared by coating fabrics on one side with plastic foams and subsequently bonding the coated fabric to a thermoplastic substrate (Belgian Patent No. 810,979).
U.S. Pat. No. 4,988,549 (Bragaw Jr. et al.) discloses a laminate comprising two adjacent layers of fluoropolymer, at least one of which is polyvinyl fluoride, the laminate being free from adhesive between the adjacent fluoropolymer layers. This laminate is produced by forming a first fluoropolymer layer, applying a second fluoropolymer layer as a dispersion to the first fluoropolymer, and coalescing the second fluoropolymer to the first fluoropolymer at a temperature below the melting point of the first fluoropolymer. This reference also discloses an embodiment wherein a reinforcing net or scrim is interposed between the two fluoropolymer layers.
U.S. Pat. No. 3,026,229 (Wilcox) discloses a method for laminating dissimilar synthetic polymers by sandwiching a composite waterleaf comprising a blend of fibrils of the polymers between the sheets prior to hot pressing, whereby each sheet is contiguous with at least some fibrils of the corresponding polymers. During the hot pressing operation, the fibrils in the composite waterleaf substantially lose their identity as fibrils, and the entire assembly becomes consolidated or fused into a compact and inseparably bonded polymeric mass.
U.S. Pat. No. 2,593,553 (Francis) discloses an open mesh textile fabric coated on each side with an organic thermoplastic film. Upon applying heat and pressure to the laminate, the films fuse to each other in the spaces between the yarns of the open mesh fabric.
European Patent Application No. 0 159 942 discloses a laminate, wherein a film is contacted to a substrate. The substrate is first treated to facilitate adhesion and bonding with the film. In one embodiment, where a substrate is laminated with a certain matrix on only one face, the substrate may be adhered to a different film matrix on its other face. The substrate may comprise woven or non-woven materials, such as KEVLAR and NOMEX.
All of these methods admit of either an unacceptable degree of costly complexity or introduce additives which limit applicability. Accordingly, it is an object of this invention to achieve a simple and direct method for laminating a fluoropolymer with a non-fluoropolymer, which avoids these shortcomings to obtain composites having improved interfacial peel-adherence.